This invention relates generally to the technology of magnetic separation, and more specifically to methods and apparatus for removal of magnetically more susceptible minute particles, often present in minor concentrations as discoloring impurities, from aqueous slurries of minute mineral particles -- such as obtained by dispersing clay, e.g., a crude kaolin clay, in water.
The iron content of commercial deposits of kaolin is generally on the order of from approximately 0.2 to 2%. Even recent publications indicate a continuing dispute as to whether the iron contaminants are in discrete form, or in a combined form within a kaolin lattice structure. While the form of this iron in clay has not been definitely established, recent evidence indicates that a portion is concentrated in or associated with nonkaolin contaminants, such as titanium oxides, etc. Whatever its form, iron contamination reduces brightness in clay, and the degree of discoloration of the clay generally increases with the amount of iron present.
In the foregoing connection, it has been known for some time that magnetically attractable contaminants can, to a degree, be removed from aqueous slurries of the aforementioned clays, by imposition on the slurry of a high intensity magnetic field. Forces produced upon the particles by the magnetic field gradient, effect differential movements of mineral grains throughout the field, in accordance with the magnetic permeability of the minerals, their size, mass, etc. The difficulties of ultilizing magnetic separation are, however, compounded by the fact that the contaminants of greatest interest are of relatively low attractability. The primary magnetic discolorant found in Middle Georgia clays, for example, is iron-stained anatase (TiO.sub.2). This impurity is very small in size, and only very weakly magnetic. Indeed, by some early views, contaminants of this general type were considered to be non-magnetic. See for example A. F. Taggart, Handbook of Mineral Dressing, page 13-02 (1960), which shows on a scale of 100, taking iron as a standard, that the relative attractability of TiO.sub.2 is 0.37.
In the copending patent application of Joseph Iannicelli, Ser. No. 19,169, filed Mar. 13, 1970, now abandoned; Ser. No. 309,839, filed Nov. 27, 1972, now abandoned; and Ser. No. 340,411, filed Mar. 12, 1973, now abandoned which applications are all assigned to the assignee of the instant application, there are disclosed method and apparatus, which in comparison to the prior art, are outstandingly effective in achieving magnetic separation of the low susceptibility impurities referred to. In accordance with the disclosure of said applications, a container adapted to have slurry passed therethrough, is filled with magnetizable elements (preferably steel wool), constituting a flux conductive matrix, which matrix serves both for diverting the slurry flow into multidudinous courses, and for concentrating magnetic flux at myriad locations therein, so as to collect the weakly susceptible particles from the slurry. The container, or canister, as it is referred to therein, is preferably of non-magnetic construction, and is disposed endwise or axially between confronting surfaces of ferromagnetic pole members, between which a magnetic field having a relatively high intensity is produced throughout the matrix. Preferably the said canister is generally cylindrical in form, and is oriented between pole members with its axis vertical, its ends being adjacent to and covered by the pole members. In the first two of the cited Iannicelli applications, the flow of slurry through the canister and matrix is in the same general direction (i.e., axial) as the high intensity magnetic field. In the last cited of the said applications, it is disclosed that certain important advantages accrue from flowing slurry through the canister in such a manner that the predominant direction of flow through the matrix is radial, i.e., from the outside diameter (O.D.) thereof, toward the axis; or from the axis toward the O.D.
In accordance with the operation of apparatus of the foregoing type, the slurry, as taught in the cited Iannicelli applications, is passed through the container at a rate sufficient to prevent sedimentation, yet slow enough to enable the collection and retention of weakly magnetic particles from the flow onto the matrix elements. The magnetic field which is applied during such collection is taught in the said applications to have an intensity of at least 7000 gauss, and preferably has a mean value in the matrix of 8500 gauss or higher. At such field strengths magnetic saturation of the matrix occurs; however, it may also be pointed out, that in the copending application of Robin R. Oder et al., Ser. No. 495,712, filed Aug. 8, 1974, now abandoned, for "Method and Apparatus for Magnetic Beneficiation of Particle Dispersion," it is disclosed that saturation need not necessarily be achieved, and that other factors as, for example, retention time in the matrix, filament size, etc., may be traded off against one another to yet yield effective results.
In any event, after a sufficient quantity of magnetics are collected, customary past practice in the art, dictates that slurry flow be discontinued, and with the field remaining energized the matrix rinsed. By the latter terms, it is meant that the matrix is initially subjected to a relatively gentle flow of "rinse" water, which serves to flow from the canister purified clay fractions still contained therein; and thereupon the matrix is de-energized and subjected to a vigorous flowing action, as by a rapid high pressure flow of water through same, in order to "flush" the collected discoloring contaminants from the matrix, which matrix is thereby regenerated and ready for further use. By and large it may be noted here, that this flushing operation has proved in the prior art to be a difficult one, as the collected particles have proved difficult to dislodge from the surfaces to which they adhere. Thus, for example, while an initial large fraction of the particles may be easily removed, it becomes increasingly difficult to remove by flushing the remaining fractions, with the last part of the adhering particles -- those secured directly to the ferromagnetic surfaces -- being most resistant to removal.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide methods and apparatus utilizable with magnetic separating apparatus of the type heretofore discussed, which serve to augment or facilitate the flushing operations used to regenerate the collection matrix, thereby improving the efficiency and effectiveness of said magnetic separation.